1. Field of the Invention
This invention relates to a secondary cell and, more particularly, to a flexible secondary cell employing a polymer electrolyte.
2. Description of Related Art
Recently, in keeping up with the tendency towards reduction in size and weight of a portable electronic equipment, there is raised an increasing demand for reducing the size, thickness and the weight of a cell supplying the power to these electronic equipment, without dependency on whether the cell is to be used for driving or backup. In addition, there is raised a demand for the possibility of efficient utilization of the housing space in the equipment. For this sort of the power source, a lithium ion secondary cell, having a high energy density or output density, may be used highly conveniently.
In particular, a polymer electrolyte secondary cell, employing a polymer electrolyte, has a feature not achieved with conventional cells, that is the feature that it is superior in resistance against liquid leakage, while being thin and light in weight, so that the cell can be designed to match to the shape of the equipment in which to mount the cell. Moreover, the polymer electrolyte secondary cell has a feature that a laminate film having an aluminum foil core as an exterior material can be used and combined with a thin sheet-like electrode and with a polymer electrolyte to prepare a thin type cell. The secondary cell employing the polymer electrolyte is being researched in order to exploit the above features proper to the cell.
Meanwhile, the polymer electrolyte cell is of such a cell structure in which the two electrodes, namely a cathode and an anode, and an electrolyte layer, are in the form of solids not containing free liquids and in which the cathode and the anode are bonded and affixed together with the adherent polymer electrolyte in-between. So, even if the electrolyte layer is a highly flaccid visco-elastic material, the electrode itself is not elastic nor extendable, and hence is locally warped and destructed due to an external force such as that caused by warping, or the layer of the active material of the electrode is detached from the current collector to lower the cell characteristics appreciably.
So, there is disclosed in e.g., the Japanese Utility Model Publication 2570162 a thin type secondary cell in which particles of an active material is affixed via an electrically conductive material to the current collector. In this thin type secondary cell, the layer of the active material of the electrode may be prohibited from becoming detached from the current collector even if the electrode is slightly flexed under an external stress. Basically, since the thin type secondary cell is of a so-called bridge structure of the two pole collectors in which the layers of the active materials of the cathode and the anode and the electrolyte layers are unified and constrained, the cell element it self is difficult to undergo flexural deformation, such that, if it is tried to flex the cell device, the device itself is fractured and destroyed. In such case, it is possible to get a liquid electrode exhibiting high fluidity interposed between the cathode and the anode to improve the slip between the cathode and the anode and hence the flexibility of the cell device. However, since it is difficult to maintain uniform contact between the cathode and the anode via the electrolyte layer, the charging/discharging reaction suffers from non-uniformities to deteriorate the cell performance such as by precipitation of metal lithium.
In the Japanese Patent No.2698145, there is disclosed a thin type secondary cell employing a sheet electrode formed of an electrically conductive polymer polymerized in a space of a rubber-like porous sheet admixed with an electrification agent. Although the electrode itself is improved appreciably in flexibility, the layer of the active material of the electrode and the electrolyte layer are sandwiched between the current collectors of the cathode and the anode to lower the flexibility of the cell device. That is, if the flexible polymer is used as a material for the electrolyte and the electrodes, the polymer is constrained by the current collector or the exterior material having a low rate of deformation or elongation, so that a cell exhibiting superior flexibility has not as yet been produced.
It is therefore an object of the present invention to provide a secondary cell having superior flexibility and high cell characteristics.
In one aspect, the present invention provides a secondary cell having a cathode, a polymer electrolyte layer and an adnode, layered together, wherein at least one of the cathode and the anode is formed by a sheet-like electrode comprised of a current collector, composed mainly of carbon fibers, and an electrode mixture carried thereon, a metal foil is provided in sliding contact with the sheet electrode on the opposite side of the sheet electrode with respect to the polymer electrolyte layer, an electrode terminal being taken from the metal foil, and wherein the cell device is sealed under a reduced pressure by an exterior member.
With the present secondary cell, in which the sheet electrode and the metal foil are physically contacted with but are not secured to each other, sufficient lubriciousness may be maintained between the sheet electrode and the metal foil to provide a structure allowing for warping deformation and which is able to cope sufficiently with external stresses.
That is, even if an external stress is applied to the present cell, the sheet electrode may be slid relative to the metal foil to permit the cell to be subjected to warping deformation to assure sufficient flexibility of the cell.
The result is that the cell is improved appreciably in flexibility to eliminate local stress concentration or distortion ascribable to the warping deformation of the cell.
Thus, with the present cell, there is no risk of the cell itself becoming locally warped and destroyed, or of the layer of the active material of the electrode becoming detached from the current collector, thus suppressing the resulting deterioration of the cell performance.
Moreover, with the present cell, in which the terminal of the electrode, made up of a current collector, composed of carbon fibers, is derived from the metal foil, the electrode structure operates as a current interrupting valve in case of overcharging or short-circuiting.
That is, in case of abnormal cell operation, such as in overcharging or short-circuiting, a large quantity of gases, evolved with heating, are preferentially accumulated between the cathode and the metal foil to interfere with contact between the cathode and the metal foil.
Since this interrupts contact conduction between the electrode and the metal foil to prevent the current from flowing, it is possible to evade the risk of thermal runaway.
Thus, the present cell, in which the above-described electrode structure has the function of a current interrupting valve, is extremely high in safety in operation.
In another aspect, the present invention provides a method for the preparation of a secondary cell including sequentially layering a metal foil, from which an electrode terminal is derived, a first electrode, made up of a first current collector, composed mainly of carbon fibers, and an electrode mixture, carried by the first current collector, a polymer electrolyte layer, and a second electrode, made up of a second current collector, carrying a layer of an active material thereon, with the metal foil being in sliding contact with the first electrode, to form a cell device, and sealing the cell device under a reduced pressure with an exterior member.
With the above-described method for the preparation of the cell, in which the first electrode and the metal foil are not physically contacted with but are not secured to each other, the cell exhibits sufficient lubriciousness between the first electrode and the metal foil and hence can be warped and deformed, while it can subsequently cope with external stresses. That is, the present method for the preparation of the cell gives a cell exhibiting sufficient flexibility in that slipping may be allowed between the first electrode and the metal foil to permit warping and deformation of the cell. The result is that the cell manufactured by the present method is improved appreciably in flexibility, while there is no possibility of occurrence of local stress concentration or distortion due to warping or deformation of the cell.
Thus, the present method for the preparation of the cell gives cell which is free from the risk of local warping or destruction of the cell itself or of the layer of the active material of the electrode from being exfoliated from the current collector to suppress the resulting deterioration of the cell performance.
The present method for the preparation of the cell gives a cell, in which the metal foil has a sliding contact with the current collector of the electrode, and in which an electrode terminal is derived from the current collector of the electrode, the electrode structure operating as a current interrupting valve in case of overcharging or short-circuiting. That is, with the cell prepared by the present method, the large quantity of gases, evolved with heating, are preferentially accumulated between the cathode composed mainly of carbon fibers and the metal foil to interfere with direct contact between the cathode and the metal foil. This breaks the contact conduction between the electrode and the metal foil to prevent the current from flowing to evade the risk of thermal runaway.
So, the present method for the preparation of the cell gives cell, in which the electrode structure has the function as the current interrupting valve to assure high safety in operation.